Emerald is a gemstone, and a variety of the mineral beryl colored green by trace amounts of chromium and sometimes vanadium. Beryl has a hardness of 7.5-8 on the 10-point Mohs scale of mineral hardness. Most emeralds are highly included, so their toughness (resistance to breakage) is classified as generally poor.
Emerald is regarded as the traditional birthstone for May, as well as the traditional gemstone for the astrological signs of Taurus, Gemini and sometimes Cancer. One of the quainter anecdotes on emeralds was by the 16th-century historian Brantome, who referred to the many impressive emeralds the Spanish under Cortez had brought back to Europe from Latin America.
On one of Cortez's most notable emeralds he had the text engraved Inter Natos Mulierum non sur-rexit mayor ("Among those born of woman there hath not arisen a greater," Matthew 11:11) which referred to John the Baptist.
Brantome considered engraving such a beautiful and simple product of nature sacrilegious and considered this act the cause for Cortez's loss of an extremely precious pearl (to which he dedicated a work, A beautiful and incomparable pearl), and even for the death of King Charles IX of France, who died soon after.
Info courtesy of Wikipedia.org